Ethylene oxide was classified as a known human carcinogen by the International Agency for Research on Cancer (IARC) and was based on its potential to induce a persistent dose-related increase in the frequency of chromosomal aberrations and sister chromatid exchange in peripheral lymphocytes and micronuclei in bone-marrow cells of workers, its ability to induce malignancies of the lymphatic and haematopoietic system in both humans and experimental animals and its ability to induce a dose related increase in the frequency of haemoglobin adducts in exposed humans and dose-related increases in the numbers of adducts in both DNA and haemoglobinin of exposed rodents. Biomarkers of effect provide an indication of critical events in the development of carcinogenesis. Markers of effect that are measured for carcinogens include end points that can be measuered in the tissue of concern with regard to the development of cancer; specifically measurement of mutations in oncogenes and tumor suppressor genes. The objective of this study is the examine mouse tumors which developed following ethylene oxide exposure for biomarkers of effect (oncogenes and tumor suppressor genes) which can be critically linked to the carcinogenic process.